This invention relates generally to electric propulsion systems for traction vehicles, and it relates more particularly to the detection of electrical overloads in such a system.
Propulsion systems for traction vehicles such as locomotives commonly use a diesel engine prime movor to drive electric generating means for supplying energy to traction motors. The power output of the generating means is regulated or varied by suitably controlling the strength of its field excitation and the speed of the prime mover. In systems such as those disclosed in U.S. Pat. No. 3,105,186--Zelina and in patent application Ser. No. 355,800--McSparran filed Apr. 30, 1973, and assigned to the General Electric Company (U.S. Pat. No. 3,878,400), the generator excitation means comprises a field winding connected for energization to an electric current supply in circuit with an intermittently conductive switch (e.g., a controllable semiconductor device), whereby the average level of excitation can be varied as desired by appropriately varying the on-off duty cycle of the field switch. This form of regulation is popularly known as time ratio control (TRC) or pulse width modulation (PWM), and its advantages are explained in earlier Zelina patents such as U.S. Pat. Nos. 2,861,237 and 2,866,944. Preferably the parameters are chosen so that under normal conditions the field switch always operates in a pulsing mode, with its per cycle proportion of "on" time being varied by the associated controls so as effectively to vary the level of excitation between predetermined maximum and minimum limits.
Such a propulsion system can be electrically overloaded if, due to a malfunction of the field switch or of its controls, full excitation current is supplied continuously to the field. In this abnormal event, the field excitation will exceed the aforesaid normal maximum limit, and as a result the dynamoelectric machines are overstressed and the diesel engine speed is undesirably reduced below that called for. If such a condition persists very long, the main components of the system will be exposed to a serious risk of damage. Accordingly, prompt and effective protective action is desired in response to abnormal overloads.
One approach to providing protection against a hazardous overload involves detecting its occurrence by sensing abnormally high values of generator current or voltage or their product. However, the hardware for doing this tends to be relatively large and cumbersome and expensive, and often its ability to perform reliably under a variety of failure modes is undesirably limited.
Another method of overload protection is to connect the intermittently conductive field switch in parallel with the field winding of the excitation means. (See the above-referenced McSparran patent.) Consequently, in the event of a fault or failure of the field switch or of its controls of a nature that causes a continuous short circuit across the field winding, all current will be diverted from the field winding, thereby unloading rather than overloading the system. While this arrangement is relatively "failsafe" and has been generally satisfactory in practice, it has several recognizable shortcomings. The person operating the propulsion system may have difficulty perceiving a loss of power caused by the described abnormality, and therefore he may fail to initiate timely corrective action. If inadvertently the field switch were continuously turned "off" as a result of a malfunction or a misoperation of its control, thereby permitting abnormally high excitation current to flow in the parallel field winding, there is no means for detecting this potentially harmful condition. Furthermore, no means is provided for differentiating between a hazardous overload caused by a continuously turned-off switch and a relatively harmless condition which may exist if the switch is only temporarily off.